CHAIN (Canadian High Arctic Ionospheric Network)

Information

The Canadian High Arctic Ionospheric Network (CHAIN) was established to study, and develop an understanding of, the effect on the Earth's environment by both short- and long-term variability of solar emissions. CHAIN has instruments located at ten stations across the Canadian Arctic: ten GPS receivers measuring phase and amplitude at a 50 Hz sample rate; and five digital ionosondes (CADI) measuring convection at 30s resolution and ionograms at one minute resolution. A sixth ionosonde installation is planned. Seven of the stations are located within the polar cap (above 66 degrees).

Having GPS receivers and ionosondes at the same location is important. We will be able to perform more realistic 4D (three spatial dimensions plus time) tomographic inversions of the ionosphere using the Multi-Instrument Data Analysis System (MIDAS) for addressing some of our scientific objectives.


The polar cap ionosphere, a region of open field lines, is, most of the time, directly coupled to the solar wind and interplanetary magnetic field. Mass and energy derived from the interaction are transported across the polar cap, and they are directly affected by the variability of solar input (both particle and electromagnetic). Because of this coupling, the polar cap ionosphere is often comprised of ionization and electromagnetic structures.

Understanding the polar cap ionosphere will help develop an understanding of the Solar Wind-Magnetosphere-Ionosphere coupling. The general scientific objectives of the CHAIN project are the understanding of:

  • Drivers and variability of polar cap convection
  • Generation and dynamics of ionization structures of the polar cap
  • Macroscale - Tongue of ionization (> 1000 km)
  • Mesoscale - Polar patches (several hundred km)
  • Microscale - Scintillation producing structures (several km)
  • Role of ionosphere in Magnetosphere-Ionosphere (M-I) coupling
  • Improve handling/modelling of effects on Global Navigation Satellite Systems (GNSS)

Canadian Advanced Digital Ionosonde (CADI)

The Canadian Advanced Digital Ionosonde (CADI) is a modern digital ionosonde, a special radar for studying the ionoised part of the Earth's upper atmosphere, the ionosphere, which is itself made up of several layers (D, E, and F - during the day, the F layer splits into F1 and F2).

An ionosonde works by transmitting short pulses of high frequency radio waves at a range of frequencies, measuring the time it takes for each frequency to be reflected, and then analyzing the result to determine the concentration (Total Electron Content - TEC) and height of each ionosphere layer. The layers are not fixed: they vary in response to changes in solar radiation and the Earth's magnetic field.

Ionosonde

The path of a radio wave is affected by any free charges in the medium through which it travels. The refractive index is governed by the electron concentration, the magnetic field of the medium, and the frequency and polarization of the transmitted wave. High frequency radio waves have some important properties when propagating in the ionosphere:

  • The refractive index is proportional to the electron concentration.
  • The refractive index is inversely proportional to the frequency of the transmitted wave.
  • There are two possible ray paths depending on the sense of polarization of the transmitted wave. This is a result of the magnetic field, which causes the ionosphere to be birefringent. The two rays are referred to as the ordinary and extraordinary components.

An ionosonde broadcasts a sweep of frequencies, usually in the range of 0.1 to 30 MHz. As the frequency increases, each wave is refracted less by the ionisation in the layer, and so each succeeding frequency penetrates further before it is reflected. The sweep frequency record produced by an ionosonde is called an ionogram.

As a wave approaches the reflection point, its group velocity approaches zero, and this increases the signal's time-of-flight. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency just exceeds the peak plasma frequency of the layer. In the case of the extraordinary wave, the magnetic field has an additional effect, and reflection occurs at a frequency that is higher than the ordinary wave by half the electron gyrofrequency.

The frequency at which a wave just penetrates a layer of ionisation is known as the critical frequency of that layer. The critical frequency is related to the electron density by the following:

  • F_c = 8.98*sqrt(Ne)for the ordinary mode.
  • F_c = 8.98*sqrt(Ne) + 0.5*Be/m for the extraordinary mode.

Here F-c is the critical frequency in Hz, Ne is the electron concentration per metre cubed, B is the magnetic field strength, e is the charge on an electron, and m is the mass of an electron. All transmitted frequencies above this critical frequency will penetrate the layer without being reflected. Their group velocity, however, will be slowed by any ionisation, and this will add to the time-of-flight. If such a wave encounters another layer, the plasma frequency of which is higher than the frequency of the wave, it will be reflected, and the return signal will be further delayed as it travels back through the underlying ionisation. The apparent or virtual height indicated by this time delay will therefore be greater than the true height. The difference between true-height and virtual height is governed by the amount of ionisation that the wave has passed through.

Measuring Ionospheric Drift using CADI

A CADI consists of a transmitter, four receivers, and the antenna system. The transmitter antenna is a delta antenna, and the receiver antenna array consists of four dipoles arranged as a square. The CADI uses interferometry to measure the ionospheric motion. Using four receiver interferometry, the CADI records the angle of arrival (AOA) and Doppler shift of the returned signal. Assuming uniform ionospheric motion in the signal illumination area, and by using AOA and Doppler shift, three components (N-S, E-W, and vertical) of the ionosphere are determined.

Drift measurements show the direction and magnitude of the convection, along with vertical drift, for one full day for Eureka (a station inside the polar cap). Convection azimuth shows antisunward convection, the pattern in the drift direction is due to the rotation of the station under the large scale convection pattern (as shown below). The location of Eureka is marked with a red dot.

GPS

CHAIN uses GSV4004B? GPS receivers supplied by GPS Silicon Valley to collect ionospheric scintillation and TEC data for all visible satellites (up to 10). The GSV4004B? uses a NovAtel? GPS702 L1/L2 GPS Antenna and a GPS receiver (NovAtel? ’s EuroPak? -3M).The EuroPak? -3M enclosure houses the GPS receiver, with modified firmware, and a low phase noise oven-controlled crystal oscillator (OCXO) that is required for monitoring phase scintillation.

The GPS receiver tracks 10 GPS signals at the L1 frequency (1575.42 MHz) and the L2 frequency (1227.6 MHz). It measures phase and amplitude (at 50-Hz rate) and code/carrier divergence (at 1-Hz rate) for each satellite being tracked on L1. Total Electron Content (TEC) is computed from combined L1 and L2 pseudorange and carrier phase measurements.

50-Hz data files are available only to members of the CHAIN science team. Data available to the public consists of 1-Hz data files in Hatanaka-compressed RINEX files at three resolutions: daily; hourly; and high-rate (every 15 minutes). The bulk of the CGSM instruments are found below the polar cap, where the magnetic field lines are closed. CHAIN enhances the CGSM by allowing researchers to study the polar cap region, where the magnetic field lines are open. As energy derived from the solar wind is transported across the polar cap, CHAIN's instruments can measure its drift and character. That CHAIN's research stations are mostly found in the polar cap, and can measure electron content, allows the CGSM to fully realize its potential.